In addition to works relying on captured images/videos by fixed surveillance cameras, unmanned aerial vehicle (UAV) based surveillance systems have been also considered. Indeed, a large number of organizations and industries are leveraging the UAV technology to automate complicated tasks. Specifically, UAVs are capable of flying at high altitudes to expand their range of view and take advantage of obstacle-free spaces. For instance, modern agriculture uses UAVs for soil health scans, fertilization, and crop monitoring. Also, law enforcement organizations use UAVs for crime investigation and crowd control, especially during large public events. In the context of COVID-19,
Somaldo et al. leveraged UAVs for social distance monitoring [1]}. Using the UAV's ventral camera, the system feeds aerial images into a trained YOLOv3-based object detector. Based on the area expansion principle, the authors developed a coordinates mapping algorithm and evaluated social distancing conditions between detected pedestrians.
Although Yang et al. managed in [2]} to achieve accurate results, their system is sensitive to the environment setting. Furthermore, the use of fixed cameras to monitor wide open areas [2]}, [4]} is not efficient due to the small covered area by a single camera. This issue is bypassed in [1]} through the use of flexibly flying UAVs. Nevertheless, the UAV's camera has generally a limited vision range, thus requiring either flying over long distances to monitor a wide area, which consumes a significant amount of energy, or deploying several UAVs simultaneously.
